1. Field of the Invention
The present invention relates generally to fiber optics. In particular, the present invention relates to the grouping of polarization maintaining fibers.
2. Background
In the field of fiber optics, one of the most valuable properties of light is the phenomenon of polarization. Light is described as a transverse wave when travelling through a medium such as glass, air or vacuum, whereby by the electric and magnetic fields which comprise the light oscillate in a plane perpendicular to the direction in which the light is travelling. Many factors may influence the polarization of light, including reflections from surfaces, external magnetic fields, and in particular, stresses in the transmitting media.
FIG. 1 shows a cut-away view of a prior art optical fiber 100. Optical fiber 100 includes a core 102 within cladding 104. The indexes of refraction of the core 102 and the cladding 104 are configured using methods standard in the art to allow light launched in to the fiber to be transported through the optical fiber 100. The core 102 and the cladding 104 are typically encapsulated in a jacket 106, which may be fabricated from materials standard in the art such as a polymer. As is known by those of ordinary skill in the art, the index of refraction of a typical optical fiber is isotropic, and thus when light is launched in to a fiber the light will tend to travel with an arbitrary polarization direction.
However, in some applications, it is desirable to have the light propagate through the fiber with a predetermined polarization. Therefore, the isotropic indexes of refraction of fibers, coupled with the fact that internal stresses in the optical fiber can influence the polarization, causes problems with fibers when used in the field. For example, during installation and use, the optical fiber may be bent and twisted, or exposed to temperature-induced stresses. Any bending of the optical fiber may change the polarization of the light travelling therein, thus influencing the final output. Furthermore, temperature-induced changes may influence the output of the fiber over time. Any such changes in the output of an optical fiber is naturally undesirable.
The prior art has solved this problem by developing polarization maintaining (PM) fibers. A PM fiber is a fiber in which the polarization planes of lightwaves launched into the fiber are maintained during propagation with little or no cross-coupling of optical power between the polarization modes. PM fibers operate by introducing a birefringence within the fiber core. Birefringence refers to the difference between propagation constant of light travelling through the fiber for two different polarizations. When birefringence is introduced into a fiber, the circular symmetry in the fiber is broken, creating two principal axes, known as the slow and fast axes of the fiber. The two axes are created in the fiber either by changing the shape of the core or by applying asymmetric stress to the core. Most PM fibers employ the stress method and are referred to as stress induced birefringence fibers. Stress applying elements in the cladding create a stress field in the core. The plane in-line with the stress field is referred to as the slow axis. The perpendicular plane is called the fast axis. The names slow and fast refer to the relative propagation velocity in each axis. The advantage of a PM fiber is that if light is launched into the fiber linearly polarized and oriented along one of these axes, then the light output from the fiber will be linearly polarized and aligned with the axis, even if the fiber is subjected to some external stresses.
FIG. 2 shows a cross-sectional diagram of one type of a prior art PM fiber 200. PM fiber 200 includes a core 202, and a pair of stress applying parts (SAP) 204 disposed proximate to core 202 within cladding 210. As will be appreciated by those of ordinary skill in the art, the configuration of FIG. 2 forms a circular SAP type, or PANDA, fiber. PANDA fibers are favored in the art since the size of a PANDA fiber is comparable to a single mode fiber. Other PM fibers that are relevant to the current invention include TIGER fiber and BOWTIE fiber, Oval-Inner clad, oval core etc. The SAP 204 are introduced to induce a constant stress within the fiber. This constant stress creates the two principal axes, shown in FIG. 2 as the fast axis 206 and the slow axis 208.
Once a PM fiber has been constructed, the quality of the polarized light transmitted through the fiber may be expressed through a factor known as the extinction ratio (ER). ER is given in dB as:ER=10 log(Pmax/Pmin)where Pmax and Pmin are the maximum and minimum signal intensities through a linear polarization analyzer as the analyzer rotates 360°. The polarization direction of maximum signal is usually perpendicular to that of the minimum signal. A one meter long patchcord constructed with a PM fiber can typically maintain an ER of 30 dB at 1,500 nanometers.
One application where a PM fiber has difficulty maintaining a proper ER is where several PM fibers must be bundled together. When PM fibers are bundled together, adjacent PM fibers may introduce unintended stresses into each other, the compounded stress field is usually not in alignment with the stress field in each PM fiber. The compounded stress field creates effective slow and fast axes for each individual fiber. In other words, the effective slow and fast axes do not overlap with the intrinsic slow and fast axes of each individual fiber. If a linearly polarized light is launched in to the fiber with its polarization direction aligned with the intrinsic slow or fast axis of the fiber, a lower ER in the output results.
Hence, there is a need for a method and apparatus which allows PM fibers to be disposed together while maintaining a desirable extinction ratio.